1. Field of the Invention
The present invention relates to a premixing-type spray gun which mixes paint and compressed air inside the atomizing head and blows out the mixture as atomized spray from the spray hole, and more particularly to a spray gun which uses a low air pressure of less than 1 kgf/cm.sup.2.
2. Description of the Prior Art
Generally, many of the spray guns called an "air spray gun" are used for spraying paint. The air spray guns using compressed air as atomizing means are generally classified into premixing and non-premixing types.
With the premixing-type spray gun, paint and compressed air are mixed inside the atomizing head thereof, and the mixture is blown out as atomized spray from the spray hole. On the other hand, the non-premixing spray gun is so constructed that the paint blow-out hole and air blow-out hole are opened out in the outer surfaces of the atomizing head and a liquid-state paint is blown out from the paint nozzle and is atomized and dispersed with another paint flow blown out as diffused mixture around the liquid-state paint flow. These spray guns utilize a suction power, that is, a negative pressure, of the compressed air for blowing out the paint. In other words, the paint is blown out under a pressure of several kgf/cm.sup.2 by means of a supply unit such as a pump and the paint will not be atomized just by the paint blow-out alone. The compressed air is used as energy for atomizing the paint. Usually an air pressure of about 2 to 5 kgf/cm.sup.2 is used for this purpose. In the premixing-type spray gun shown by way of example in FIGS. 6A and 6B, a paint blow-out hole e opens through an air cap b, and a nozzle hole d in nozzle 3 is opened and closed by a needle valve c to control the blow-out of paint. Under the action of compressed air supplied around the nozzle hole d, the paint is dispersed inside the air cap b and sprayed out from a spray hole e opening out of the center of the air cap b and which is opposite the nozzle hole d.
Generally, the spray hole e is circular, but the shape of the nozzle can be elliptical or rectangular according to the spraying conditions. Because the spray particle size is larger than that with the non-premixing-type spray guns, the spray guns of this premixing type are used as special spray guns for spraying wall paint on a building, adhesive, etc. where the sprayed surface may not be smooth.
On the other hand, the non-premixing-type spray guns used as a so-called "spray gun" and suitably usable with a variety of paints and spraying conditions have been proposed. The common features of these well-known spray guns lie in that they have a blow-out hole in the paint nozzle at the center thereof, the blow-out hole being directed outwardly of the atomizing head. There are provided around this blow-out hole circular air orifices so that compressed air is blown out surrounding a paint flow blown out from the blow-out hole. Namely, the paint and compressed air are blown out separately and mixed and atomized in front of the atomizing head. Normally, many of the spray guns of this type have a pair of lateral air orifices across the atomized paint, which blow compressed air onto the atomized flow from both sides thereof, thereby adjusting the shape of the spray pattern.
Therefore, the spray flow blown out as a circular pattern can have the shape adjusted so as to be flattened by changing the blow-out pressure at the lateral air orifices or the blow-out amount. Since the paint and air are mixed outside of the spray gun, the sprayed paint particle size can be made smaller as the compressed air amount (pressure) is increased.
In conventional spray guns, however, atomization is done with air, which will result in the problem of paint scattering. This paint scattering causes considerable problems in spray efficiency, environmental sanitation or public health, etc. More particularly, the paint will be scattered even more with a higher air blowing pressure. Thus, it is desired to spray the paint with an air pressure as low as possible. For a better finish of the paint-sprayed surface, however, the particle size of atomized paint has to be smaller, which requires a pulverization of paint under the action of highly compressed air. These two factors are quite opposite to each other. With the majority of conventional spray guns, paint spraying is done with much paint scattering and low spraying efficiency as well as at the cost of environmental security.
However, the effective utilization of resources and the security of the environment will be increasingly important for human beings, and so a spray gun which produces atomization under lower pressure will be necessary.
The atomizing medium for paint is not limited to air. For example, it is already known to atomize paint by blowing it out under a high pressure from the nozzle into the atmosphere so that the paint will collide with the latter. However, industrial spray guns of this type require a paint pressure as high as 100 kgf/cm.sup.2. For this purpose, a special pump is required, and also a danger may possibly exist. To avoid such problems, a spray gun is also used in which the paint pressure is lowered to several ten kgf/cm.sup.2 and compressed air is used for the reduced pressure. However, the problems still are not eliminated, and the air pressure is not sufficiently low. That is to say, no satisfactory spray gun has yet been proposed.